Michael A. Covington    Michael A. Covington, Ph.D.
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Replacing a tuning-eye tube
Two-transistor static protection for shortwave antenna input

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A despicable episode in Valdosta history
The lynching of Mary Turner and others in May, 1918


I have just been aghast to learn of a shameful episode in the history of my home town, Valdosta, Georgia. If I had been paying attention, I would probably have heard of this 1918 atrocity in 2010, when a historical marker was erected and the newspaper covered it. But I didn't hear of it growing up, nor later, even though I've kept up some interest in Valdosta history. In fact, a historian mentioned in Wikipedia found the county historical society unaware of it in the 1990s, even though it had been well publicized back in 1918.

I refer you to Wikipedia for the gruesome story, which is too nauseating to retell. Suffice it to say that Mary Turner was not just killed; it was much worse than that. And although there had been, and continued to be, other lynchings in the area, what made this one particularly diabolical is that Ms. Turner was not even accused of a crime.

It is disconcerting to realize that when I was a boy in Valdosta, about 40 to 50 years later, there must have been many people alive who remembered the incident, and some who had participated in it. But it was never mentioned and is not in the two books about Valdosta history that I know of. Only once during my adolescence do I remember an adult mentioning that there had been lynchings in the area, and that was completely without details.

But this may explain why there was fear of "race riots" in the late 1960s. People alive at the time knew full well that their predecessors had done things that might provoke violent retribution. In particular, I remember a weekend in 1968 or 1969 when rumors were flying, but there was no sign of an actual riot. It occurs to me now that it might have been the 50th anniversary of the lynching rampage.

It also makes the town's fixation on high-school football more excusable. Football was the only local institution that undeniably and obviously benefited from racial integration. It may have been the thing that kept a racial powder keg from exploding.

By the way, I can take some solace that none of my ancestors were in Lowndes or Brooks County in 1918 (my father was from farther north, and my mother, from farther west), so they, at least, were not involved.


Short notes

I wrote about a short skirmish with the flu on December 2 and then didn't write anything else for eight days — you're probably imagining I've been down with the flu.

Not so. I'm fine, but Melody is having some back trouble and I am again the family's driver and errand-runner. That, on top of a thriving software business, keeps me busy.

I've posted nothing about societal or political matters for a while, since the situation is too volatile, but I do have one exhortation for anyone following the impeachment: Follow the facts. Everything depends on what people actually did, in fact about rather fine details of what they did, and also on facts about the laws. And I would add that a fair trial is one that is decided on the facts, not political pledges made years earlier. What would we think about any other kind of trial in which the jury had taken sides long in advance?

In other news, I shifted my attention from electronics projects to some infrastructure modernization. I found a place in my workshop where a 20-amp circuit had been extended with #14 (15-amp) wire, and I need an outlet added, and elsewhere there were extension cords that need to be replaced by permanent wiring. We're going to have an electrician in, but first I need to scout everything out. And there is also a possibility that my darkroom will be operational in a few weeks, for the first time since four years ago. Or not, depending on how busy I am with other things!


Two-transistor overvoltage protection for shortwave radio antenna input

A two-transistor bidirectional Zener clamp makes a very good "static protector" for the input of a shortwave radio. It protects against static electricity and also voltages induced by thunderstorms in the area (which can be hundreds of volts even if not very close to a lightning strike). I added one to my antenna tuner, so its complete circuit is now what is shown below. The red components are added to the classic antenna tuner circuit.


Unlike the traditional pair of diodes, the bidirectional clamp doesn't turn on until it reaches about 8 volts; that means it won't be affected by strong signals from nearby AM stations or power lines. But, unlike a Zener diode, it has very little leakage and little capacitance. The MPF102 input transistor in my radio can tolerate 25 volts, so an 8-volt clamp is sufficient, I think.

You can see that I also made the resistor termination switchable. The reason? With the resistors in place (switch closed), the antenna picks up less noise but is also less tunable; it loses a lot of sensitivity at frequencies other than the one it is cut for. With the resistors removed (switch open), it is much more tunable. For more about this, click here.

Here's what the two-transistor clamp looks like, assembled:


And here's what my ancient Tektronix transistor curve tracer tells me about the threshold voltages (2 volts per division):


Note: I would consider selling this instrument (local pickup in Athens, Georgia, only; it weighs 75 pounds). It is fully functional but hasn't had any substantial maintenance in several years. Contact me if you want to make an offer.

Flu express

This notebook entry is the only thing I've produced today (for anyone) because I'm taking a sick day. Last night I appeared to have the flu, but only for eight hours or so, though there is a lingering slight fever. I've had this experience in several recent years; maybe the vaccine enables me to fight off the viruses with only a short battle. Or maybe it's something else.

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Replacing a tuning-eye tube


Happy new month! And a blessed First Sunday in Advent to all.

If you didn't see yesterday's entry about the noble art of electronic repair, and why the repair business is a lot like the insurance business, click here and read it. Then proceed...

On Saturday I did something in my workshop that is rarely done in this century. I replaced a tuning-eye tube.

As you can see, it made a dramatic difference. But what is this contraption?

It's a vacuum tube that glows green on one end, and part of the green glow is blocked off, depending on a signal voltage applied to it. Above, you see it as the display device on my somewhat souped-up Heathkit capacitor tester.


Back in the 1930s, radio manufacturers wanted to include a meter in the radio, to help people tune each station exactly. (Tune for the highest meter reading.) But meters are expensive, and there would also need to be a tube to amplify the signal for the meter. Further, the purpose of such a meter is just to find the maximum — it doesn't have to have a calibrated scale.

So someone had the idea of building an indicator device into the amplifying tube itself. The "tuning eye" is what resulted. It looks rather like an eye, hence the name.

By the 1960s, tuning eyes were rare; everybody got good local radio reception and didn't feel the need for a tuning aid. Shortwave radios for serious listeners had meters, not tuning eyes. So the technology was almost obsolete...

But tuning eyes hung on in a few kinds of test equipment, notably capacitor testers. My Heathkit IT-11 has one. And what I didn't know, when I bought that instrument about a quarter century ago, was that its tuning eye was on its last legs. It was hard to see.

I can put up with a dim display for 25 years, but not forever. So I replaced the tube. Here are the old and new ones, for comparison. The phosphor in the old one has deteriorated and turned brown. The new one (about 60 years old, but never used, and fresh in its RCA box from an eBay vendor) is tremendously brighter.


There is an excellent video about eye tubes at this link, for Patreon subscribers to "Mr. Carlson's Lab." My recollection is that you can subscribe for one month for $2, well worth it.

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